I also recommend to launch in a single market, but not to wait too long before launching elsewhere. My experience was that we launched in a medium size city and wanted to refine too much the model so it would be perfect for other cities. It turned out that that
specific market was not the best one for us, as soon as we decided to launch in another market it took off and just until then we were able to really refine the business model.
In short, don't try to be everywhere when launching, so you can be really close to the activity and make quick fixes, but also don't be shy to launch in other places as soon as possible.